Galileo Galilei had discovered four moons orbiting Jupiter, includi

OFFICIAL EXPLANATION



• HIGHLIGHTS

Use past perfect!

→ BY THE TIME X, Y is a Big Signal: use past perfect.
This sentence uses that structure. The structure is reversed.


This sentence is a variation on "By the Time X, Y."
I will simply flip and rewrite the sentence:
By the time Christiaan Huygens discovered Titan, the largest moon of Saturn, in 1655,
Galileo had discovered four moons orbiting Jupiter, including Ganymede, the largest moon in the solar system.

By the Time X, Y is a signal to use past perfect.
Time X marks off a discrete point in the past.
Before that time, Y events happened.
To talk about the past of the past, we use past perfect, namely
HAD + PAST PARTICIPLE [verbED]


• I see no errors. Keep. (See Notes, below.)


• which refers to an antecedent noun, and in this case, improperly refers to Jupiter
-- Jupiter does not include Ganymede. The four moons do.
-- but we know that which must refer [absurdly and incorrectly] to Jupiter because the verb includes is singular.
-- the verbal "being" is unnecessary
Eliminate B

• Um, Jupiter is definitely not the largest moon
(I hear the Indigo Girls' "Galileo" play in my head. (Linked below.) Levity is good.)
Eliminate C

• we need past perfect [had discovered] to talk about "the past of the past."
-- in order to emphasize that Galileo was far ahead of Huygens, we use had discovered (not just discovered)
• The verb "discovered" is incorrect.
Eliminate D

• again, wrong verb. We need to use had discovered, not discovered, as is used here. Same problem as that in D
Eliminate E

The best answer is A

NOTES

-- Some words and phrases indicate a time shift.
One such phrase is
By the time X, Y.
Verbs must shift to reflect that time shift.

The Y events happen before the X event.
Write X in simple past tense.
Write Y in past perfect (had verbED).


--By the time X, Y is fairly common on the GMAT.

When we see the phrase in a question involving the past, almost without exception,
(1) the Y events come before the X event,
(2) Y events take past perfect, and
(3) the X event takes simple past.

This sentence is a good example of contexts in which we should use past perfect (had verbED) for the earlier (Y) events rather than simple past.

By the time means "when" or "at that time."
Huygens made his discoveries at a discrete, defined time.
Huygens' discovery is the X event.

The X event rendered in simple past tense (discovered) creates a "stop right here" marker of something that happened before now.
Then we use past perfect to describe the Y events before that marker.

By the time X, Y.
By the time THIS thing happened (simple past), THIS other thing had happened (past perfect).

By the time means when or at THIS particular point in time.

We have one moment at which THIS thing happened in simple past tense. (Huygens discovered a moon of Saturn.)
-- Before that discrete moment in time there were other events, namely: Galileo had already discovered four moons of Jupiter.
-- Simply because of the way that English works, the specific language used in past perfect "announces" that we are speaking about things that happened BEFORE the event later or latest in time.

• Past perfect construction

If two or more events happen in the past and at least one of them is later in time than the other(s), we use simple past tense to describe the later-in-time event, and past perfect to describe the events that happened before that final event.

Past perfect is often described as the past of the past.

Past perfect, active: HAD/HAVE + past participle (verbED)

Past perfect, passive: HAD/HAVE + been + past participle [not in play in this question]

Requirements? To use past perfect
(1) at least one event must be rendered in simple past tense (or we must know sequence from a time marker or stamp such as by last Friday night, and
(2) often NO time sequence words such as after, before, or subsequently are present.

COMMENTS

ravigupta2912 , welcome to SC Butler.

As always eakabuah and itoyj , I am glad to have your contributions.

I wish I could have met Galileo.
I make whimsical lists . . . such as "The 10 People I Wish I Could Have Met."

Then my lists grow. I think I am near 100 persons.
But Galileo is Top Ten.

Well done, everyone. Kudos to all.